1. Field of the Invention
The present invention relates to plasma reactor systems, and in particular relates to a method of and system for monitoring the impedance in a parallel-plate plasma reactor system.
2. Discussion of the Background
Ionized gas or “plasma” may be used during processing and fabrication of substrates (e.g., semiconductor devices, flat panel displays and other products requiring etching or deposition of materials). Plasma may be used to etch or remove material from or sputter or deposit material onto a semiconducting, conducting or insulating surface. Creating a plasma for use in manufacturing or fabrication processes typically is done by introducing a low-pressure process gas into a chamber surrounding a substrate that resides on a substrate support member, more commonly referred to as a “chuck.”
In a capacitively coupled plasma reactor system, an electrode connected to an RF power source resides above the chuck. The molecules of the low-pressure gas in the chamber are ionized into a plasma by activating the radio frequency energy (power) source and heating electrons once the gas molecules enter the chamber. The plasma then flows over and interacts with the substrate, which is typically biased by providing RF power to the chuck supporting the substrate. In this regard, the chuck serves as the lower electrode, and is sometimes referred to as a “chuck electrode.” The plasma gas flowing over the chuck is removed by a vacuum system connected to the chamber.
One of the key factors that determines the yield and overall quality of the plasma processing is the uniformity of the plasma process at the surface of the substrate. In a capacitively coupled plasma reactor, the process uniformity is affected by the design of the overall system, and in particular by the physical relationship of the upper electrode, the chuck, the plasma generation source, and the radio frequency (RF) tuning electronics. Improvements that lead to the ability to control reactor process uniformity are critical to manufacturers of plasma reactors and are the focus of significant efforts.
The ability to control plasma process parameters in a capacitively coupled plasma reactor depends to a large degree on the proper measurement of the plasma conditions. The plasma parameters, including the plasma density, the electron temperature, the impinging ion energy distribution, etc., must be monitored to produce reliable process results for advanced plasma processing systems. Those parameters are normally termed as internal parameters. Internal parameters can be monitored and used as a feedback to vary the external control process parameters (“system control parameters”), such as RF power, gas flow rate, gas pressure, the RF power and frequency, DC bias, etch chemistries, etch time, electrode spacing, wafer placement, and the like.
Because of the problem of plasma disturbance and contamination introduced by some plasma measurement techniques, only non-intrusive plasma monitors are used in the semi-conductor processing industries. There are presently several different non-intrusive techniques available to measure plasma properties. One such technique is optical emission spectroscopy, wherein light emitted by the plasma is collected and spectrally analyzed to extract the plasma properties. However, this technique has some serious shortcomings, such as low measurement reproducibility of emission line intensity, and lens degradation.
Another technique involves monitoring the RF voltage and current on the electrodes. The relative phase difference can resolve the real system impedance and provide useful information about the plasma parameters. However, this technique is often hindered by the small phase difference involved in the measurement. The substrate and the electrodes contribute a large fraction to the real system impedance, while the plasma impedance is usually only a small perturbation (<10%) of the total system impedance. Even with this limitation, these RF monitors are still used widely in semiconductor manufacturing, as well as by the equipment tool manufacturers in advanced process control (APC) systems.
Some plasma parameter measurement attempts have been made in APC systems by correlating the passive RF measurements with certain process parameters, such as the so-called equipment footprint, the etch or deposition rate, the end-point of the pattern etch, process clean end-point, etc., to deduce the control functions or traces and establish a correlation with the level change in the discharge impedance measured by the passive RF measurements. However, this correlation method requires a large number of measurements for every individual system to obtain statistically averaged plasma characteristics.
There are other problems with known plasma measuring techniques. For example, certain passive conventional monitoring techniques involve measuring the current and voltage of the RF power provided to the upper electrode to form the plasma. However, this technique is problematic because the plasma reacts to the RF power signal, which can result in a change of the plasma state. Other techniques involve the use of fundamental and harmonics signals produced in the plasma to detect the state of the plasma. However, it can be difficult to obtain meaningful measurements when noise interferes with the low-amplitude RF signals.
Further, in most plasma monitoring methods, the impedance of the plasma is determined by measuring the current, voltage and the phase difference between the two at the fundamental frequency (or the first few harmonics) of the RF power source. The impedance contains both imaginary and real parts. The real part is related to the resistance R associated with the circuit itself (called the circuit resistance) and of the plasma (called the “plasma resistance”). The imaginary part of the system impedance is due primarily to the capacitance C of the plasma sheaths near the electrodes, particularly for frequencies less than the plasma discharge resonance (when the plasma impedance is purely resistive); below which the plasma is capacitive in nature and above which the plasma is inductive in nature. Therefore, at the low harmonics (i.e. 2nd, 3rd, . . . ), the complex system impedance is approximated by Z=1/jωC+R, with 1/ωC>>R. Therein, the resistance R mostly comprises circuit resistance. Thus, it is usually rather difficult to determine the real part of the system impedance due to the large phase angle or nearly singular argument, and the difficulty of measuring thereof. Moreover, the difficulty of extracting a small plasma resistance from a relatively large circuit resistance further exacerbates the problem.